This disclosure is directed to a tuner useful in antennas. It particularly finds an application in an antenna system where it is physically not possible to display an antenna constructed to an optimum antenna length. As a generalization, it is desirable that an antenna have length which is some multiple of 1/4 of the wavelength. At frequencies above 100MHz, this poses no particular problem because the wavelength is relatively short. This is a much more severe problem at lower frequencies, and it is particularly difficult below about 1MHz. Using that frequency as an example, a 1MHz vertically positioned quarter wavelength antenna requires a height of 75 meters to provide good radiation efficiency with an easily matched impedance in the coupling circuit. For the ham operator in the 10 meter band, antenna size is only then approaching that which can be readily handled.
As a generalization, deviation from multiples of a quarter wavelength and especially where the antenna length is significantly less than about one quarter of the wavelength, coupling problems arise. For one, there is more difficulty in matching the impedance with the coupling circuit. In addition to that, radiation efficiency is severely impacted. As another factor, the impedance looking into the antenna terminals becomes more reactive for antennas which are significantly less than one quarter wavelength. While it is possible to provide an impedance matching network connected between the transmitter and the antenna, those are not so easily implemented because there are relatively large currents or voltages involved, particularly where the transmission involves substantial power. One of the factors involved in antenna construction is the use of an antenna assembly which is formed of several serially connected sections. They are connected with some type of switch mechanism. The switches are used to either short or disconnect sections of the antenna. Those switches however normally require some type of switch control relay or other mechanism. This connects the switch as well as the switch control mechanism in the current flow power directed to the antenna assembled from one or multiple sections. It is of course important to isolate the power being delivered to the antenna to those operative sections which make up the antenna at the selected configuration; otherwise, the switch and switch control mechanisms may become activated in some sense with spurious radiation from the antenna system so that even a switch which has been switched off may become energized in an undesirable fashion.
Switch isolation can be obtained by placing RF chokes in the switch circuitry. This is not desirable because they interfere with the field resultant from the multiple sections of the circuitry making up the switched antenna. There is another problem too, namely the RF chokes create inductive fields which cannot be quickly switched. Further, such circuits involve control lines which are normally connected to the switches. The control lines themselves become active constitutents of the antenna even when this is not intended. In effect, they represent a circuit having continuity involved in the antenna elements and/or support structure. They are sources of parasitic field radiation. Moreover, they are a source of leakage around such an open switch.
The present disclosure sets forth a system for handling this problem. This will be described in the context of an antenna system formed of multiple sections which operates at selected wavelengths. In that context, the antenna system will typically be operating at substantially less than the optimum quarter wavelength. Moreover, rapid antenna switching is accomplished by the structure of the present disclosure which particularly assists in holding down spurious radiation involving the switching and coupling circuits.